Feeling the heat

05/12/04

Successful 'first light' for the Mid-Infrared VISIR instrument on theVLT

Colour composite of the Galactic Centre.
Click here for a full size version of this image.

Close to midnight on April 30, 2004, intriguing thermal infrared
images of dust and gas heated by invisible stars in a distant region
of our Milky Way appeared on a computer screen in the control room of
the ESO Very Large Telescope (VLT).

These images mark the successful "First Light" of the VLT Imager and
Spectrometer in the InfraRed (VISIR), the latest instrument to be
installed on this powerful telescope facility at the ESO Paranal
Observatory in Chile. The event was greeted with a mixture of delight,
satisfaction and some relief by the team of astronomers and engineers
from the consortium of French and Dutch Institutes and ESO who have
worked on the development of VISIR for around 10 years [1].

Pierre-Olivier Lagage (CEA, France), the Principal Investigator, is
content : "This is a wonderful day! A result of many years of
dedication by a team of engineers and technicians, who can today be
proud of their work. With VISIR, astronomers will have at their
disposal a great instrument on a marvellous telescope. And the gain is
enormous; 20 minutes of observing with VISIR is equivalent to a whole
night of observing on a 3-4m class telescope."

Dutch astronomer and co-PI Jan-Willem Pel (Groningen, The Netherlands)
adds: "What's more, VISIR features a unique observing mode in the
mid-infrared: spectroscopy at a very high spectral resolution. This
will open up new possibilities such as the study of warm molecular
hydrogen most likely to be an important component of our galaxy."

From cometary tails to centres of galaxies

The mid-infrared spectral region extends from a few to a few tens of
microns in wavelength and provides a unique view of our Universe.
Optical astronomy, that is astronomy at wavelengths to which our eyes
are sensitive, is mostly directed towards light emitted by gas, be it
in stars, nebulae or galaxies. Mid-Infrared astronomy, however, allows
us to also detect solid dust particles at temperatures of -200 to +300
deg C.

Dust is very abundant in the universe in many different environments,
ranging from cometary tails to the centres of galaxies. This dust also
often totally absorbs and hence blocks the visible light reaching us
from such objects. Red light, and especially infrared light, can
propagate much better in dust clouds.

Many important astrophysical processes occur in regions of high
obscuration by dust, most notably star formation and the late stages
of their evolution, when stars that have burnt nearly all their fuel
shed much of their outer layers and dust grains form in their "stellar
wind". Stars are born in so-called molecular clouds. The proto-stars
feed from these clouds and are shielded from the outside by
them. Infrared is a tool - very much as ultrasound is for medical
inspections - for looking into those otherwise hidden regions to study
the stellar "embryos".

It is thus crucial to also observe the Universe in the infrared and
mid-infrared. Unfortunately, there are also infrared-emitting
molecules in the Earth's atmosphere, e.g. water vapour, Nitric Oxides,
Ozone, Methane. Because of these gases, the atmosphere is completely
opaque at certain wavelengths, except in a few "windows" where the
Earth's atmosphere is transparent.

Even in these windows, however, the sky and telescope emit radiation
in the infrared to an extent that observing in the mid-infrared at
night is comparable to trying to do optical astronomy in
daytime. Ground-based infrared astronomers have thus become extremely
adept at developing special techniques called "chopping' and "nodding"
for detecting the extremely faint astronomical signals against this
unwanted bright background [3].

VISIR: an extremely complex instrument

VISIR - the VLT Imager and Spectrometer in the InfraRed - is a complex
multi-mode instrument designed to operate in the 10 and 20 micron
atmospheric windows, i.e. at wavelengths up to about 40 times longer
than visible light and to provide images as well as spectra at a wide
range of resolving power up to ~ 30.000. It can sample images down to
the diffraction limit of the 8.2-m Melipal telescope (0.27 arcsec at
10 micron wavelength, i.e. corresponding to a resolution of 500 m on
the Moon), which is expected to be reached routinely due to the
excellent seeing conditions experienced for a large fraction of the
time at the VLT [2].

Because at room temperature the metal and glass of VISIR would emit
strongly at exactly the same wavelengths and would swamp any faint
mid-infrared astronomical signals, the whole VISIR instrument is
cooled to a temperature close to -250 deg C and its two panoramic
256x256 pixel array detectors to even lower temperatures, only a few
degrees above absolute zero. It is also kept in a vacuum tank to avoid
the unavoidable condensation of water and icing which would otherwise
occur.

The complete instrument is mounted on the telescope and must remain
rigid to within a few thousandths of a millimetre as the telescope
moves to acquire and then track objects anywhere in the sky. Needless
to say, this makes for an extremely complex instrument and explains
the many years needed to develop and bring it to the telescope on the
top of Paranal. VISIR also includes a number of important
technological innovations, most notably its unique cryogenic motor
drive systems comprising integrated stepper motors, gears and clutches
whose shape is similar to that of the box of the famous French
Camembert cheese.

VISIR is mounted on Melipal

The fully integrated VISIR plus all the associated equipment
(amounting to a total of around 8 tons) was air freighted from Paris
to Santiago de Chile and arrived at the Paranal Observatory on 25th
March after a subsequent 1500 km journey by road. Following tests to
confirm that nothing had been damaged, VISIR was mounted on the third
VLT telescope "Melipal" on April 27th. ESO PR Photos 16a/04 and 16b/04
show the approximately 1.6 tons of VISIR being mounted at the
Cassegrain focus, below the 8.2-m main mirror.

First technical light on a star was achieved on April 29th, shortly
after VISIR had been cooled down to its operating temperature. This
allowed to proceed with the necessary first basic operations,
including focusing the telescope, and tests. While telescope focusing
was one of the difficult and frequent tasks faced by astronomers in
the past, this is no longer so with the active optics feature of the
VLT telescopes which, in principle, has to be focused only once after
which it will forever be automatically kept in perfect focus.

First images and spectra from VISIR

The photos above resulted from some of the first observational tests
with VISIR. PR Photo 16c/04 shows the scientific "First Light" image,
obtained one day later on April 30th, of a visually obscured star
forming region nearly 10,000 light-years away in our galaxy, the Milky
Way. The picture shown here is a false-colour image made by combining
three digital images of the intensity of the infrared emission from
this region at wavelengths of 11.3 micron (one of the Polycyclic
Aromatic Hydrocarbon - PAH - features), 12.8 micron (an emission line
of ionised neon) and 19 micron (cool dust emission).

Until now, an elegant way to avoid the problems caused by the emission
and absorption of the atmosphere was to fly infrared telescopes on
satellites as was done in the highly successful IRAS and ISO missions
and currently the Spitzer observatory.

For both technical and cost reasons, however, such telescopes have so
far been limited to only 60-85 cm in diameter. While very sensitive
therefore, the spatial resolution (sharpness) delivered by these
telescopes is 10 times worse than that of the 8.2-m diameter VLT
telescopes. They have also not been equipped with the very high
spectral resolution capability, a feature of the VISIR instrument,
which is thus expected to remain the instrument of choice for a wide
range of studies for many years to come despite the competition from
space.

Source: Eurekalert & others

Last reviewed:
By John M. Grohol, Psy.D. on
21 Feb 2009
Published on PsychCentral.com. All rights reserved.